Catalytic reforming of naphtha feedstock to is an important, well known process in the petroleum refining industry. Most naphtha feedstocks have low octane numbers because they contain large quantities of naphthenes and paraffins. In catalytic reforming, these components go through a variety of hydrocarbon conversions to produce in a gasoline product having a higher octane number.
Some of the more important conversion reactions include dehydrogenation of naphthenes to aromatics, and dehydrocyclization of normal paraffins to aromatics. Less desirable reactions which commonly occur include hydrocracking of paraffins and naphthenes, which produces low-value gaseous hydrocarbons (also referred to as “light ends”) such as methane and ethane. Due to these less desirable reactions, an important objective of catalytic reforming is to rearrange the structure of the hydrocarbon molecules to form higher octane products without any significant change in the carbon number distribution of the stock (e.g. to minimize the formation of light end products).
Referring to FIG. 1, in a typical reforming operation, the process is carried out in a series of reforming reactors (R1, R2, R3). Each reforming reactor is generally provided with a fixed bed of catalyst that receives and catalyzes vaporized naphtha in the presence of hydrogen, at an elevated pressure and temperature (“reforming conditions”). Feed heaters (H1, H2, H3) disposed between the reactors because the reforming reactions are endothermic. The effluent from the last reactor (R3) is separated into a liquid product and a hydrogen-rich vaporous effluent. The vaporous effluent is used as recycle gas in the reforming process.
Catalytic reforming processes use catalysts having one or more dehydrogenation-promoting metals dispersed on a porous support, such as chlorinated alumina. During the reforming operation, the activity of the reforming catalyst gradually declines due to the build-up of coke (typically over a period of 5-24 months), and the temperature of the reactor must be raised to compensate for the loss in catalytic activity. Eventually, economics dictate the necessity of interrupting the reforming process in order to regenerate the catalyst. At the end of run, naphtha reforming operations are discontinued, feed is removed from the reforming unit, and the reforming catalyst is regenerated in-situ using a specific regeneration and startup procedure.
In a conventional catalyst regeneration processes, the catalyst is regenerated in situ (e.g. in the reactor without moving the reforming catalyst). To regenerate a reforming catalyst in situ, the following sequential steps are typically employed. (See, Antos and Aitani, Catalytic Naptha Reforming, 2nd Edition, 2004, pages 433-457).
1. Reactor Purge: After the reforming operations are discontinued and feed is removed from the reactor, an inert gas (e.g. recycle gas, nitrogen) is used to purge/remove residual hydrocarbon material and hydrogen from the reactor.
2. Coke Burn: Coke deposited on the catalyst is burned, typically at temperatures of greater than 725° F. (385° C.), in the presence of a gas consisting of an inert gas such as nitrogen, and air (as an oxygen source needed to burn off the carbon).
3. Catalyst Rejuvenation (Oxidation): The catalyst hydrogenation metals often agglomerate as a result of sintering during the coke burn step. The metals are re-disbursed by subjecting the catalyst to chlorine under a full-air atmosphere.
4. Oxygen Purge: Following rejuvenation, the system is purged of water and oxygen. This step is traditionally conducted at low temperature (400° F., 204° C.) and low pressure (<50 psig, <446 kpa) using an inert gas such as nitrogen.
5. Metals Reduction: The dried catalyst is subjected to a hydrogen-containing reduction gas to reduce the metals. During this stage, the reactor temperature is raised to about 700-900° F. (371-482° C.).
Following the period of time during which the catalyst undergoes oxidation in order to remove coke on the catalyst, the reactor is depressurized, and the catalyst is extensively cooled, typically to about 400° F. (204° C.). Then, prior to catalyst reduction, water and oxygen are thoroughly purged from the catalyst bed(s) by circulating an inert gas through the beds over a period of typically at least several hours. During the metal reduction step, the catalyst is initially contacted with hydrogen at the cooled temperature, and the reactor temperature is then increased, typically by at least about 400° F., to promote catalyst reduction.
The time required for cooling the reactor to around 400° F. (204° C.), and the time required for re-heating the reactor to reducing conditions, both add a total additional twenty-four (24) hours or more to prior art catalyst regeneration processes. In addition, the pressure drop during this step necessitates the reforming unit compressor be turned off.
In addition, during regeneration, halogens (e.g., Cl) present in the system form corrosive by-products, such as HCl. During this entire time for cooling and then re-heating the reactor, the reforming equipment is typically unprotected from the corrosiveness of HCl. Conventional methods for regenerating reforming catalyst typically take at least several days to complete, during a substantial portion of which the reforming equipment is susceptible to damage by the corrosive by-products.
There is a continued need for improved processes for reforming catalyst regeneration that are effective, less time-consuming, simpler, and which minimize the potential for damage to the reforming equipment, thereby providing major operating economies for integrated hydrocarbon reforming/catalyst regeneration processes, as compared with processes of the prior art.
US2010/0152021 to Lew ('021 to Lew), published Jun. 17, 2010, describes a reforming catalyst regeneration process wherein following the rejuvenation step, the reactor is cooled from about 950° F. (510° C.) to preferably 850° F. (454° C.) with the compressor running. The nitrogen “purge” decreases the oxygen content to around 1-2 volume percent oxygen. The pressure of the reactor is decreased to a preferred 70 psig, then hydrogen is injected, combusting the oxygen. Thereafter, the reactor pressure is increased to a preferred 170 psig. By raising the drying stage temperature and pressure, this eliminates the need to turn off the compressor, and shortens the time needed to raise the reactor temperature to the temperatures needed for the reduction step.
As noted above, when introducing hydrogen following the drying step described in '021 to Lew, the residual oxygen is combusted. Because the combustion of oxygen in the presence of hydrogen is an exothermic reaction, it was thought necessary to both reduce the oxygen content to below 0.5 volume percent, and limit the temperature rise while the oxygen is being consumed (by limiting the amount of hydrogen being introduced).
It has now been found that by initiating the metals reduction step before achieving a 1-2 volume percent oxygen level, and by introducing the hydrogen at a higher rate, results in improved catalyst performance once the catalyst has been fully regenerated.